Study of phase transition of two-dimensional ferroelectric copolymer P(VDF-TrFE) Langmuir monolayer by Maxwell displacement current and Brewster angle microscopy

Phase transition of ferroelectric Langmuir monolayers composed of poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) copolymer with various VDF ratios, was investigated. The thermodynamic, dielectric, and structural properties of these two-dimensional ferroelectric systems were analyzed by coupling surface pressure-area isotherms, Maxwell displacement currents (MDCs), and Brewster angle microscopy (BAM) images. A low-temperature phase transition, specific for the ferroelectric monolayer, was found out with an order-disorder phase transition. The phase transition temperature was dependent on the composition ratio of the copolymer, and the Gibbs free energy reached maximum at this temperature. A negative MDC peak representing dipole moment oriented to the subphase was observed during monolayer compression and disappeared above the order-disorder transition. Interestingly, the BAM images showed decrease of domain size with increase of temperature. These results were well explained based on the order-disorder transition. (C) 2009 American Institute of Physics. [doi: 10.1063/1.3212945]